I don't know of any that will just fit inside the phone. They have all these ones that take up space outside the phone, as you mentioned, but a single SIM phone is meant to just hold and use a single SIM.
You could try looking at these dual SIM phones...
... Samsung Galaxy Note 9
And of course you would have to find one that works with the carrier/service provider you wish to use.
Verizon is not allowing CDMA-only phones anymore because they plan to shut it down at the end of the year, and Sprint the following year. One would think any GSM LTE phone would be OK but they say the only allow models they have tested and it is not simply a matter if the primary LTE bands are supported. Moreover, normally Dual-SIM phones service a lot of bands, but unlike the Note 8, the Note 9 only has a few and would only be useful in India and Singapore, so the Note 9 dual-SIM is useless. I don't want the small battery off the Note 8. I need Colombia, South America, and the US. The Verizon Note 9 has bands for about everywhere, and for sure works on the Verizon network. The AT&T could work too, but it is also single-SIM, and I don't know if it works with Verizon. If it does, I might go that route, but it is still single-SIM.
One would THINK an interrnal dual-SIM add-on would be available somewhere because the phone already comes with an adapter that has a SIM and conbination 2ndSIM/SD slot that can be used with a second SIM on their Dual-SIM phone. I would think a vendor could easily run a short ribbon between them, but I haven't seen anything like that either. Actually, I would think Samsung would implement it themselves since the slot is already there.
Thanks!