I did not have a bench with my loom and after using a dining chair for a while I knew I would need better seating when I started to weave the tartan for my kilt project.
Luckily I had saved a long plank of weathered barn board pine that was 2 inches by 12 inches and about 7 foot long. I cleaned off most of the weathered wood with a hand plane and then finished with a belt sander. I cut the board into the size pieces I needed and then attached the "feet" and the crossbar with mortise and tenon joints. All joined surfaces got wood glue, some spots got galvanized screws - others got finishing nails.
The seat was made from a piece of 1 inch by 12 inch pine shelving reinforced on the underside and rectangular holes cut to let it drop down on the uprights. Each side upright got 4 pairs of holes drilled to hold wood dowels set into wood blocks to provide for support and adjustment.
I grabbed the two plastic baskets and made a wire "loop" that lets them hang on the uprights and sits on the outside of the seat to provide a place to keep bobbins, shuttles and little scissors at close reach.