I’m looking at my frame thinking I can get a bigger bottle on using a strap on side entry cage, but needs to be low profile mounting, there is an SKS not side one but seems to sit quite far away from the frame.

You can get strap on cage mount bosses and fit whatever bottle cage takes your fancy.

I’m wondering if a side-entry cage might just push the strap-mounts round when you try to insert a bottle.  Will prob still go in but you might end up repositioning and that’d piss me of after a while

Sounds like the OP wants something that holds the bottle as close to the tube as possible, which strap on mounts with a standard cage bolted to it won’t achieve.

Kelvin, yes that. Its currently zip tied to the shock mount.

Could something like the Fabric cageless bottle or Fidlock offer a solution?

No bottle mounts on mine either I went with ‘r3pro’ 3d printed mount. works great even mounted under my downtube.

Does that fit any bike? And does the tape stay stuck? Looks useful.

MT ZOOM do a good one, but I think you’ll need bolt holes: https://mtzoom.com/collections/cages-and-bottles/products/dual-sided-side-pull-cage

So far so good.