I'm not sure whether this is an original idea or not, but I thought I'd share a little jig/technique I used to cut some grooves for splines to reinforce some carcass mitres on large panels. I used this for stopped grooves, but it would work equally well for full length cuts. It's easiest to explain with a few pics...

Here's the jig set up with a panel in it:

IMG_20190602_094027.jpg

it's essentially just a piece of 18mm mdf on the bottom with a mitre at the end, and two sides, also mitred, with a piece of 6mm mdf screwed on to the mitres so the edge is just below where the groove needs to be. The panel being routed sits hard up in against the 6mm mdf and is clamped in place. The router is fitted with a template guide and a straight bit (6mm in this case), with the template guide riding on the top of the 6mm mdf and the router base sitting flat on the face:

IMG_20190602_094212.jpg

I left a gap between the edge of the panel and the side of the jig to start the router off in, and here's the result:

IMG_20190602_094947.jpg

It's quick, accurate and repeatable.