Pelmet blocks

Does anyone recommend fixtures other than Screwfix assembly blocks for fixing pelmets to wall cabinets?

I believe the done thing is to

1) attach the blocks to the back of cut length pelmet,
2) clamp pelmet to cupboard with quick clamps
3) screw assembly block to underside of cabinet.

Any tips on spacing between each block and screw lengths needed - I think my carcass and pelmet are 18mm thick.

Thanks in advance.

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