Searching elsewhere on the forums showed that some have used a Citizenship Certificate to return at the land border. Searching on the government websites shows this should be acceptable. (As long as it's issued and delivered by Canada Post in time)
https://www.cbsa-asfc.gc.ca/travel-voyage/td-dv-eng.html
Acceptable documents to denote identity and citizenship
Upon arrival at a Canadian port of entry, travellers must satisfy a CBSA border services officer (BSO) that they meet the requirements for entry into Canada. For Canadian citizens, permanent residents and persons registered under the Indian Act, this can be done through questioning and through verifying documentation such as a:
- Canadian passport
- Canadian birth certificate
- permanent residence card
- citizenship card
- Secure Certificate of Indian Status (SCIS) card or valid Certificate of Indian Status (CIS) card
If you do not have a passport, and are returning to Canada, the following documents can denote
identity and citizenship:
- NEXUS card, held by a Canadian citizen, when entering Canada by air (when coming from the U.S.), land, or marine modes
- FAST card (Free and Secure Trade), issued to a Canadian citizen (when arriving by land or marine modes only)
- Canadian emergency travel document
- Canadian temporary passport
- Certificate of Canadian citizenship (issued from 1954 to present)
- Enhanced driver's license issued by a Canadian province or territory
- Enhanced identification/photo card issued by a Canadian province or territory
Permanent residents of Canada who are members of the NEXUS or FAST programs must travel with a passport and proof of permanent residence, and may be asked to present these documents to the officer upon arrival at the border.