Given the ease of CRISPR/Cas9-mediated genome editing, why not epitope-tag your protein by using an oligonucleotide + Cas9 + sgRNA to insert the necessary codons directly into the gene? Then you can have access to pre-existing high-affinity antibodies for epitopes such as MYC or FLAG.
It may be better than trying to make a new antibody. That involves more steps, takes more time and the mAb you get may not work for all applications.
The Nonet lab developed a monoclonal antibody tool kit for worms, including some to ER proteins (http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0010161). The antibodies are available at the Developmental Studies Hybridoma Bank (http://dshb.biology.uiowa.edu/). We’ve also had success with mammalian antibodies (to Calnexin for example), so those can be worth trying if the conservation is high.
Many mammalian ER-resident proteins have at the C-terminus the ER-retention signal -KDEL or variants. Long time ago I looked for
such proteins in C. elegans and I found that there are more proteins with HDEL than KDEL. You can use the comercially
available anti-KDEL or HDEL antibody. I used the a-HDEL from Santa Cruz in IF and I was not very satisfied, but it might depend on
protocol and tissue.