As of 2023 (unofficial source, but unfortunately official sources for Laos are often non-existent or useless), VOA is available at the following Thai-Lao border crossings:
- Chiang Khong – Houixay, Chiang Rai to Bokeo Province;
- Nakaxeng – Kaenthao, Loei to Sayabouly Province;
- Houi Koln – Nam Ngeun, Nan to Sayabouly Province;
- Lao-Thai Friendship Bridge I, Nongkhai-Vientiane Capital (eVisa available);
- Tha Naleng train station in Vientiane;
- Lao-Thai Friendship Bridge II, Moukdahan-Savannakhet Province (eVisa available);
- Lao-Thai Friendship Bridge III, Nakorn Phanom-Khammouane Province;
- Chongmek – Vangtao, Ubol Ratchathani to Champasack Province.
The bridges and train options are obviously busy and popular, and so is the one at Chiang Khong (they've already built the 4th bridge here). I've never been to Kaen Thao, but apparently there's a bridge there as well, while Chongmek/Vangtao is close-ish to the major city of Ubon Ratchathani.
So this leaves the Houi Koln – Nam Ngeun crossing (ห้วยโก๋น - ນ້ຳເງີນ), which looks pretty promising: it's deep in the mountains and the nearest towns on both sides (Chiang Klang, Pak Beng) are pretty minor and off the beaten track. It's probably still not going to be as small as your MY/TH crossing, since the smallest Lao entry points don't issue VOA or allow third-country nationals.
Update: The Lao Embassy in Australia has an undated different list of checkpoints (PDF) that allow VOA, and it's shorter:
- Borkeo Province: Houayxay (Lao) - Xiengkhong (Thai). Be able to obtain Visa on arrival
- Khammoun Province: Thakhek (Lao) - Nakhonphanom (Thai). Be able to obtain Visa on arrival
- Savannakhet Province: Savannakhet (Lao) - Mukdahan (Thai). Be able to obtain Visa on
arrival
- Champasak Vangtao (Lao) - Xongmek (Thai). Be able to obtain Visa on arrival
- Parksane (Lao) - Beungkane (Thai). Be able to obtain Visa on arrival
Plus VTE, LPQ airports and Friendship Bridge, which support e-visa as well. Note the absence of Houi Koln – Nam Ngeun on this list :/