Hi,
I made my own from scrap wood so as to mill small printed circuit boards, and used a small vane type vacuum pump to get a good vacuum. All the unused exposed holes I seal with clingfilm (ceramwrap in USA?) and or masking tape so that I can get a reliable 0.8 - 0.9 bar pressure difference when working. With your 1" square parts that would amount to some 12lbs holding down force.
I milled the top surface flat whilst in position, and have measured the overall flatness to be better than 0.001" - very necessary when milling pcb tracks 0.01" wide.
The vacuum pump is a cheap Chinese unit similar to this :-
SC 420/2, Industrial VFD spindle from StoneyCNC
UC100 + UCCNC
Cut2D, Autosketch10, Draftsight, Eagle 9.5.1
There is no problem, however simple, that cannot be made more complicated by thinking about it.