In the past i have just made a flat plate that covers two large holes in the plenum.
A hole is drilled in each end of this plate for studs that are screwed into the plenum to go through.

Use some soft motorbike valve springs or similar and nyloc nuts on the studs to set the required blow off pressure.

Of course there is a sheet of rubber between the plenum and plate aswell.

As you know, its only there incase of a backfire.

Cheep, Easy and very effective!