Keypad Backing Thickness - Vacuformed Enclosure

[MudAndSnow]

I want to make an enclosure for a 4x4 membrane keypad out of ABS or similar material.  The enclosure will be slightly larger than keypad and shaped to have rounded edges using vacuum former.

The abs sheets are surprisingly expensive, specially for thicker ones. Which makes me want thin sheets but of course not so thin that it flexes noticeably when user presses buttons.

Any ideas what an ideal thickness would be?

If you have other suggestions please feel free to share.

Keypad:
https://www.amazon.com/WINGONEER-Universial-Switch-Keyboard-Arduino/dp/B06XHFC19G/ref=pd_aw_vtph_421_tr_2?_encoding=UTF8&pd_rd_i=B06XHFC19G&pd_rd_r=067f9398-d8e3-11e8-943e-3f50e486d3b4&pd_rd_w=89K8S&pd_rd_wg=nUgTi&pf_rd_i=mobile-dp-sims&pf_rd_m=ATVPDKIKX0DER&pf_rd_p=61cf5fd0-cfc9-447f-a425-f14760645161&pf_rd_r=A4M7MGENPZNFE2RV4VRR&pf_rd_s=mobile-dp-sims&pf_rd_t=40701&psc=1&refRID=A4M7MGENPZNFE2RV4VRR

ABS sheets:
https://www.amazon.com/Thickness-FYBREX-Textured-Breakage-Resistance/dp/B074NZ4TK1/ref=mp_s_a_1_3?ie=UTF8&qid=1540533377&sr=8-3&pi=AC_SX236_SY340_FMwebp_QL65&keywords=abs+sheet&dpPl=1&dpID=41ihs%2BqUmxL&ref=plSrch