sintra and styrene , wall, hang
Sintra is produced in both 3mm and 6mm thickness.....the latter being approx. 1/4" thick would be ridgid enough for your size. If you had a wide panoramic, it would require a "backer" board like Gatorfoam or even MDF board or coroplast.
Yes, a border ON THE PRINT would be the preferred look with the edges of the border going right to the edge of the Sintra. For effect, the edges of the sintra can be beveled. Technique-wise, adhesive-mount the entire print with a 1/4" bleed and then trim the excess...and of course, slap-on a UV laminate.
If you are "stuck" having to use 3 mm sintra, then a recommended backer would be the gatorfoam....use the 1/2" gatorfoam as a backer and cut the backer to 2" smaller than the sintra on both dimensions. Then center the sintra onto the smaller backer....in that manner, the backer remains "hidden" from the viewer. Using wood or MDF is OK, but both are much heavier than the gatorfoam. I've used coroplast in the same manner and it is very strong and extremely lightweight....but it has the problem of nothing can adhere to it....expensive mounting tape is required.