Make edges perpendicular

I want to mount this piece of plywood in such a way that it is reasonable removable.

I first need to true up the edges of the wood and that is difficult because of it's small size.

I have a table saw, circular saw, and reciprocating saw.

I would like to avoid the table saw because of its weight and bulk.

It is also not mounted on a stand.

How can I true up the wood?

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Deja Vu all over again !

Sharpen your pencil ... you OK with that ?
Use a rule & straight edge .. with me so far ?
Lay out your size & shape.. I know this part is really hard.
Use a hand saw to cut carefully along the line. Aunt Viola ! I knew you could do it ! We're all so proud .. John T.

You must not be a teacher. :-)