if you don't want to buy a starnut, here's what i would do.
if you are right handed, assemble the fork, headset and stem, leaving the two stem bolts loose. push down, hard, on the stem while holding the stem straight WRT the front wheel. with your right hand tighten one of the stem bolts sufficiently to hold the stem in place. take a breath, then tighten the other bolt. check for slop. if not satisfied, loosen the stem bolts and try again.
it might take a few attempts to get it right. and bouncing the front end of the bike off the ground while holding the stem in place, between attempts, may eliminate a consistent looseness.
this process, if successful, does not compromise in any way the security of the headset/stem interface. the top cap and bolt may be attached, after the fact, for cosmetic reasons, if preferred.