The wrapper is permissible; the holdings make it halal.
Hold halal-screened funds inside
HLAL, SPUS, Amana funds, or Wahed inside your 401(k) or IRA. Avoid the default target-date fund glide path that includes conventional bond exposure.
Capture the employer match
The match itself is permissible compensation. Set your contribution to at least the match level so you do not leave it on the table.
Roth vs traditional
A Roth wrapper is permissible. A traditional wrapper is also permissible. Pick by tax planning, not Sharia rules.
Purification
Some halal funds publish purification rates; subtract the rate from dividends and donate to charity without intent of reward. The /tools/purification-calculator handles the math.