That is a nice-looking set of blocks and stuff in your photo.
Perhaps the best magnetic square I have used is a small magetic chuck with having a double face rail, and squared up to near zero. *Most of my square grinding has been /is just clamping to a block. A block is clean and so little chance of bugs coming with it. Solid blocks can be cleaner than ones with holes.
You set the add-on mag on your surface grinder chuck so have the horizontal with your surface grinder chuck and the vertical positioning with your small add-on chuck.
For squaring off a row of parts the add-on chuck is the berries.
The common solid magnetic square for about $130 to $250or so can be handy if doing a lot of grinding but often you can get the small chuck for that price...and the squaring mag and the add on chuck may have places for bugs to follow.
A higher grade surface plate and a square checking device/set-up are first needed along with true tested square blocks, some very good clamps, an indicator that can be attached to your wheel head.
Pairs of shim stock can be handy so to go between, and to correct a square error. Parallel clamps can be better than C clamps, but C clamps are often OK.
I have used gun blue to mark my best side of blocks. I would check any bought blocks and shoot for 12 millionths for my good side. And set of JoBlocks, a 45* block, and a bar diamond dresser for dressing a wheel for edge bevel
Here is a double rail magnet chuck:
Brown & Sharpe Model 510 Permanent Magnetic Chuck 5" x 10" | eBay