Reddit mentions of NEMA 5-20P to 2X 5-20R Y Splitter Cord - 6 Foot, 20A/125V, 12/3 AWG - Iron Box # IBX-2725-06

I would not use L6. You don't need the locking ability, it will just cost you more, and you won't be able to use duplex outlets, which will increase your costs even more. Use regular 6-20. This outlet: https://smile.amazon.com/Leviton-5822-I-Receptacle-Commercial-Grounding/dp/B000U39UY4/ for example.

I got some ordinary 110V C13 Y-cables (i.e. one regular 110 plug, two C13 computer connections). Cut off the 110v plug, and replaced it with a 220v plug. (The C14 part is the same for both voltages - you can get them already made that way, it was just more expensive for some reason, so I made my own.)

This: https://smile.amazon.com/NEMA-6-20P-C13-Splitter-Cord/dp/B00CP6BYFC/ if you don't want to make your own. (There are probably cheaper ones, I didn't look very hard.) Edit: actually these: https://smile.amazon.com/dp/B00CP63IX8/ since you need 12AWG wire, not 14.

That way you can run 4 devices per duplex outlet. (Up to around 4amp each.)

/u/mikexli said an L3+ consumes 4.5A, so that's perfect - 4.5 * 4 = 18, which fits perfectly in a 20A circuit.

You don't need a PDU, that just increases your costs. Simply plug the L3+ in the way I describe and you are good.

Have the electrician run 1 circuit, with 6-20 duplex outlet, per 4 L3+. It's MUCH cheaper than running say, 1 50A circuit, or 30A, and using a PDU.

If you buy the outlets (electricians don't typically stock them in their truck, although they can get them if you tell him ahead of time), then the electrician can use ordinary 12/2 cable which they have mountains of, to wire it. This will save you money. Make sure to tell him to do a dedicated circuit for each outlet.