Reddit mentions of Strenco 1 Inch Self Adhesive Hook and Loop - 5 Yard Set - White Sticky Back Tape Fastener - Light Weight

Here's what I used:

Buttons/joystick kit: https://www.amazon.com/dp/B06WGX76HT/ref=cm_sw_r_cp_apa_i_e3u4DbNFYM9PT
Video adapter: https://www.amazon.com/dp/B07ZVMWPZP/ref=cm_sw_r_cp_apa_i_B4u4DbC4ARRDN
CanaKit Raspberry Pi 3 B+ https://www.amazon.com/dp/B07BC6WH7V/ref=cm_sw_r_cp_apa_i_u6u4Db97MWE57
Pi 3b case: https://www.amazon.com/dp/B07349HT26/ref=cm_sw_r_cp_apa_i_v5u4Db1FS3T1H
Audio amp: https://www.amazon.com/dp/B07C1Q1FPT/ref=cm_sw_r_cp_apa_i_.6u4DbW8KHPC5
Adhesive Velcro (to mount the various items): https://www.amazon.com/dp/B00PKQUS4W/ref=cm_sw_r_cp_apa_i_4.u4DbNJGANFX
1'1/8 Spade bit:
https://www.homedepot.com/p/Bosch-Daredevil-1-1-8-in-x-6-in-Spade-Bit-DSB1015/203274560

General thoughts on the modding process... I had never done something like this, never used a raspberry pi either. I followed ETA's tutorial, browsed this forum for info and everything went smoothly. I encountered only 2 minor difficulties.
The button/joystick kit had 1 joystick wire that was faulty and thus was impossible to tell from looking at it.. After mounting and connecting everything the stick would not register 1 direction. Thinking the stock was at fault I ended up ordering a second kit. It took some tinkering to eventually narrowing it down to the wire! Drilling the 2 extra holes was rather straightforward but I did not think of how the wires would reach the buttons! I ended up drilling 2 small holes in the bottom of the plastic cover - not a super neat job but it's all hidden anyway.
Bottom line, it's relatively simple but I think any DIY project can have unexpected difficulties.. so be prepared.

Tips for anyone who wants to give this a try.. .
Is it easy? I would say yes. The components are plug and play so it's essentially Ikea furniture mounting level. But.. it does require to do things you likely have never done before so take your time and don't rush. The drilling part does require some minimal manual skills. I wouldn't try drilling through the main board... in particular if your equipment is some lightweight stuff. That main board holding the controls is rather thick, and you would risk damaging the artwork if you mess up. Cutting through the Plexi also seems rather challenging.
Cutting through the thin board placed under the controls however is simple.
One advice, if you plan to put back the black plastic cover that protects the underside of the controls, you need to keep that in mind when you decide to position your extra buttons. If you place them too high, you won't have enough clearance for the hidden part of the buttons due to the presence of the plastic cover. So, either position these lower (as I did) or plan not to use the cover.. it might be possible to cut through the black plastic cover but that sounds like a lot of trouble if you ask me..
Side note, I painted the back panel in black because the back of the cab is a little visible in my setup... It looks pretty good.

The cost...
Well, let's be honest, this is not a cheap project. Sure, if you scored a 75$ cab you're off to a good start but to me the look factor was essential. I personally think the Marvel cab is the best looking one currently available in the A1up lineup for a 2 player setup.
So, 250$ for the cab at Walmart. The Pi, the case, video adapter, SD card... Roughly 200$
Total.. 450$

Conclusion...
I absolutely love this thing... But not for the reasons I initially thought. The gaming part ends up being less important for me than the look. I have it setup in my game room (got a pool table and a second cab) and I find myself just turning the cabs on for the mood more than to play with them. Also, the modded cab with its led buttons looks just gorgeous.

That's it, happy modding everyone!