[Dave]

If you can make threads its easy then!
Make youre base, makins sure you leave a 1cm square at each corner with no water channels in.
Cut some 5mm+ thick plexiglass to just larger than the top of the block. Place it atop the block and mark out where you want to drill the holes for mounting.
Drill them at 6mm. Sit the plexi atop the block again and then mark the centres of these hole throught onto the block - they should line up with the 1cm square corner bit with no water channels. Drill holes down these centers to 5mm, drill down about 2/3rds throught the block. Tap these threads with a 6mm tap.

Drill the holes for your hose barbs in the plexiglass lid.

Run a fine bead of silicon (the clear stuff works best) round the outside edge of the block, then place the lid atop it and screw it down, not fully though, leave about 2mm thick of silicon. let this dry, then tighten the top right down..