I've tried multiple litter boxes with my Siberians and they've always preferred the Iris Top Entry. Not the best for older cats, but my cats at least will always choose this one over a front entry box regardless of size:
https://www.amazon.com/dp/B01MRV5YNO/ref=sspa_dk_detail_1?psc=1&pd_rd_i=B01MRV5YNO&pd_rd_w=NNOvM&pf_rd_p=48d372c1-f7e1-4b8b-9d02-4bd86f5158c5&pd_rd_wg=3ZvwI&pf_rd_r=KJW681JPAP2BXAYN9Z1C&pd_rd_r=1df8542c-2367-438e-848c-9434d22822de&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExTFo0UTNDVzZQSzdWJmVuY3J5cHRlZElkPUEwMjQxODM3MjE3U1lBNFVKNThLTyZlbmNyeXB0ZWRBZElkPUEwNjg4MDc0Mzg1S05BQjVEV1VSUSZ3aWRnZXROYW1lPXNwX2RldGFpbCZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

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Dr. Elsey's Kitten Attract Litter was also incredibly helpful when box training them initially:
https://www.chewy.com/dr-elseys-kitten-attract-clumping/dp/28981