Crop videoplane

Hello, I am trying to replicate the Crop effect in Adobe Premiere. I have something that works using alphamasks but it seems a little overkill and I'm wondering if there is any optimized way to do this (shaders?).

I'm using four matrices loading four images of a black line (left, right, bottom, top). Matrices are moved using jit.rota and combined with jit.concat. The resulting matrix is sent to a jit.gl.texture and this texture goes to the right inlet of a jit.gl.slab @file cc.alphaglue.jxs.
Everything works but there is probably an easier and more efficient way to do this, right?