Board:
8-ply alphacellulose -- Or 2x4-ply boards vacuum-wetmounted together. Sanded edges.
Mount for cross stitch:
Insert Attach-EZ nylon T's about 1/2" apart all around perimeter of fabric, as far in from edges as practical. Then position the fabric on the board, and lace the nylon T's together across the back, in both directions, with light cotton thread.
This works like speed-laces on boots. Laces may be tensioned & retensioned as needed to straighten the weave of the fabric on the board.
I say this method is better than lacing or pinning, and here's why:
A) The stress of each nylon T is distributed over three or four parallel threads of the weave. But thread puts all of its stress on just one perpendicular thread of the weave.
B) Lacing may be done with lighter thread, which provides a more tension-limiting mount. If anything should happen that could tear the weave, the lightweight speed-laced threads would break.
C) As fast & easy as pinning, but with all the benefits of traditional lacing.
NOTES:
1. This method is suitable for coarse-weave fabrics. It is NOT suitable for fabrics with a fine weave, which could be damaged by installation of the nylon T's.
2. I recommend dulling the needle of the Attach-EZ tool with emery cloth before using it on fabric. Otherwise, careless use could cut the weave, but for cross stitch mounting, there is no need for it to be sharp.