You could stick a sheet of whatever plastic you're using down to the bed and mill slots/holes for the air
iv considered it for long runs, might be a pain to get the holes routed to each o-ring if its a complex pattern

the problem with mixing diffrent size o-rings is the diffrent surface arias would compress the o-rings diffrent amounts

With a sufficiently big syringe how many pads do you think could be served? Or are you sucking the air out of one at a time and swapping the syringe round?
im going to try using one syringe and fix one of the one way valves as an exsaust so you could pull the initial vacuum the pad valves will hold the vacuum ill press the syringe home and the air iv removed from the pads will be routed through the exsaust one way valve ?????? if you can get your head around that :)

i need to do a drawing to show you what i mean.... imagine you have two one way valves attached to the end of the syringe, youll work it out :)

am i sending you a large syringe ? (iv got a few from work)